Variable frequency response circuit



May 13, 1941. I. ZAKARIAS I VARIABLE-FREQUENCY RESPONSE CIRCUIT Filed A ril 16, 1940 INVENTOR. v {)mre. %akari aa ATTORNEY.

Patented May 13, 1941 UNITED STATES r.v -.,0 FFIca Era-lite: f 1 VARIABLE mumc esmnsemc Imre Zakarias, Budapest, hangar-y, as'signor to d o Patents Corporation, acor'porationof f NewYork f3. inpputmonapfii 16,1540, serial-Natas a:

' jlnliungai'y May 6, 1939 I 'Ih'e present invention relates to frequency variation response circuits for converting. a

variable frequency, including both the frequency of, an alternating'current or potential and the natural or tuning frequency of a resonant cir- ,1 cult, into energy having an amplitude varying.

proportionately to said frequency variations.

More particularly the invention relates to frequency variation response circuits of the type disclosed in'my copending application Serial No.

146,180, filed June 3, 1937 entitled Frequency, variation response circuit, now Patent No.

2,208,091, comprising substantially an electron discharge tube having at least two control electrodes or grids for controlling the electronspa'ce current passing from the cathode toithe anode of the tube. Means are providedwhereby an alternating, such as a high frequency potential impressed upon the'first grid near the cathode causing a corresponding variation or modulation of the normally steady electron space current will cause a potential at thefrequency of the im'- pressed potential to be induced uponthe second grid by reactive coupling with the electron current known as space charge or electron coupling as distinguished from the ordinary physical couplings by means of an inductive, capacitative, or resistive impedance. Such an electron coupling may be obtained by providing a positively biased screen betweenthe control grids causing a concentrated electron space charge or "virtual cathode to beset up near the second control grid and resulting in a capacitative coupling'of th latter with the space'current fluctuations.

According to said copending application a tuned circuit is connected between the second grid and cathode wherebythe phase of the induced potential on the second grid due to space charge coupling with the electron current varies in either direction from a normal quadrature phase relation, when said tuned circuit is in resonance with the impressed potential on the firstgrid, proportionately to the relative frequency departure of the impressed potential from the frequency to which said tuned circuit is resonant. By mutual intermodulation of the relatively phase shifted potentials on the grids there is developed in this manner in the output .current of the tubes. component havingan amplitude varying proportionately to the relative frequency departure of the impressed potential from the frequency to which said tuned circuit is resonant. This component may be segregated from the output'current by the aid of a suitable coupling impedance or network and applied to .a desired utilization circuit such an an audio frequency amplifier followed by a loud speaker in the case of a frequency modulation receiver, or

to an electrically controlled tuning element of 5 the local oscillator in a superheterodyne receiver if the circuit is used as a discriminator in an automatic frequency control system embodied in a radio receiver in a manner well understood .by those skilled in the art.

Anobject of the presentin-vention is the provision of an improved frequency variation respouse circuit of the above type adapted to staibilize and enhance the dependability of the frequency conversion.

Other objects and advantages of the invention will become more apparent as the following description proceeds taken with reference to the accompanying drawing forming part of this specification, and wherein:

variation response-"circuit accordingto the in vention, T y

Figure 2 is'a circuit diagram' showing a modification' of Figure 1,

Figures 3 and4 are characteristic curves explanatory of the function of the invention,

Figure5 is a diagram showing a further modification especially suited'for-converting variations of the natural or tuning frequency of a.

resonant circuit. I

Like reference numerals identify like parts in the diiferent'views of the drawing. 7

The space charge coupling utilized in systems of the above character is by its very nature a uni-lateral capacitative coupling i. e. a coupling directed from the first grid near the cathode to the second control grid shunted to cathode through the tuned circuit having a negative sense compared with an ordinary capacitative coupling effected by means of a physical condensenjor capacity element. The value of the space charge coupling in electron tubes is of the order of about .1 to 10 mmfd.; i. e. sufficient to afford eflicient conversion or demodulation of a varying frequency signal as. disclosed in the above mentioned copending application.

A disadvantage of the system disclosed in said application whichmay be objectionable in certain cases and tend to impair the dependability and stability of the system is the fact that undesirable reaction between the grids due to the inter-electrode capacity between the grids (of the order of .2 mmfd.) may result in distortions of the output signals and other disadvantages.

515 According to the present invention this diffi- I Figure 1 is a diagram of'a simple frequency.

tb neutralize the uni-lateral space charge cou-a 5 pling between the grids resulting in a new unilateral coupling directed opposite to the original space charge coupling. In"certain Eases, espe:-*'

cially for short waves, a resistance is advanfirst control grid I2. uni-lateral coupling is utilized according to the invention to effect a frequency conversion in a manner described in the following.

,The input alternating potential such as a high frequency potential is applied by way of input terminals ab to the grid I3 and cathode II. A tuned circuit comprising an inductance coil 20 shunted by a condenser 2| and a variable retageous ly placed in series with the coupling consistance 20" is connected between the grid I2 denser to equalize the effect of the transit time of the electrons passing from one grid to the other. Thus, if the space charge coupling is neutralized in the above described manner, the

and ground I9, whereby the potential induced in the grid I2 by the input potential on the grid I3 will have a phase varying in dependence upon the relative frequency departure of said imnew uni-lateral coupling produced between the pressed potential from the tuning frequency to grids in a sense opposite to the original space charge coupling will be in a direction fro second grid towards the first gridlocated, ear the cathode. r

According to the present invention this new 2 the which the circuit 2Il2I is resonant. Thus, assuming that the input potential is a frequency modulated signal received from a frequency modulation broadcast transmitter and that the ciro cuit -21 is. resonant to the carrier frequency uni-lateral coupling is utilized for effecting a of thissignal, the phase of the potential induced frequency conversion by applyin the signalpctential to the second grid remote from the mu ode and providing a tuned circuit connected by inter-changing thecircuits connected to? the control grids in the arrangement disclosed in the copendins pplication. In this mariner a pure uni-lateral oupling; is, obtained between the action, as the inter-electrode capacity between "the grids may .form a part of a neutralizing stability and dependability of the system.

electron tube comprising a cathode, II followed in succession by a first control grid I2 and a second control grid I3, a screen grid I4 are ranged between the grids I2 an I3, and'a plate prising a resistance by-passed by a condenser connected in the return lead from the cathode I I toground I8 or any other zero potentialpoint of the-system according to standard practice.-

If in a tube of this cathode" is pnoduced\ near the control grid I3 resulting in a negative capacitative or space.

capacity,.thus resulting'in increased operati'enal on the rid I2 byQthe carrier frequency will be in qua ature to the phase of the input signal on the grid I3 due to the capacitative character the flrstgrid near the cathode or in other? words, on the space charge coupling, and will varyain either direction from this normal phase relation proportionately to the frequency deviation'of the input signal with respect to the carrier frequency.

As a result, there is produced in the output or grids substantially without objectionable inter: plate circuit of the tube II) by mutual intermodulation of the potentials on the grids I2 and I3, a component varying in sign and magnitude in proportion to the relative phase departure from a normal quadrature phase relation be- Referring to Figure 1, item III represents an 5 tween the potentials on the grids; i. e., in turn in proportion to the frequency variations of the input signal. Thus, for the carrier frequency the potentials on the grids I2 and I3 will be in quad rature resulting in a zero inter-modulation prodor anode I5. Item I6 is a biasin networkcom 40 not; i. e., the anode current will be equal to its normal steady value. If the input frequency increases above or decreases below the resonant frequency of the circuit 20-2 I, the relative phase of the grid potentials will vary accordingly in type a S a P en s respect to the normal quadrature phase relation, impressed upon the' grid I2 near the cathode, a" concentrated variable space charge or virtual resulting in an increase or decrease of the steady inter-modulation current in the output circuit as c ar e c p in a c ion from the s i I2 in ut frequency ,f impressed upon the grid and to the grid I3. This negati e space charge 0 upling according to the present invention is neutralized by a positive capacitative coupling by the provision of a condenser Il connecting the\grids I2 and I3 and preferably having a variableresistance I8 connected in series therewith fer equalizing the effect of the transit time of the electrons passing from, the grid I2 to grid I3, especially in the case of short waves as pointed out hereinabove. In this manner, by neutralizatiop of the negative space charge coupling by a new uni-lateral coupling is producedin the opposite direction; i. e., from the grid I3 to the grid I2. This phenomenon will be further understood from the following: Inasmuch as the space charge coupling from the first control grid I2 toward the second co'ntrol grid I3 is uni-lateral, and the coupling by means of the condenser I1 is bi-lateral or mutual, the neutralizing of the uni-lateral space charge coupling by the bilateral condenser coupling' will result in a new uni-lateral condenser coupling which is directed opposite to the original space charge coupling,

corresponding positive capacitative coupling, a

iao epresents the steady normal plate current. for he unmodulated or carrier frequency je. Curve b shows the same relation if the damping of thejtuned circuit 2 2I is increased such as by decreasing the shunt resistance 20'. As is seen, the steepness of the curves 0 and b at opposite sides from the frequency is determines the amplification or conversion efficiency which may be controlled by adjusting the resistance 20 to 0 suit existing requirements. The frequency responsive output currentmay be extracted or segregated in any desired manner such as bymeans of, filter in the plate circuit comprising a series resistance 24 and shunting condensers 1 26 and 21 and applied to a subsequent utilizai. e. from thesecond control grid I3 towards the denser 23 in accordance with standard practice.

In addition to the plate current variations as shown at a and b in Figure 3, similar variations but in an opposite sense occur in the screen grid current as shown at cand d, respectively, in

Figure 4, wherein the screen grid currents}; is of the circuit39-40. If the tuning of the latter is maintained at a fixed value, variations of the plotted as a function or the frequency f of the impressed input potential and im represents the normal screen grid current for the input (carrier) frequency f0. According to a further modification of the invention the screen grid current is utilized as the'output or demodulated current as shown ,in Figure fi which difiers from Figure 1 merely by the provision of a low-pass filter 29-40-41 in the screen grid circuit and a coupling of the output terminals cd with the screen grid by means of the coupling impedance 22 and coupling condenser 32.

According to a further modification, the circuits described in Figure 1 and 2 may serve for converting the tuning frequency of the resonant circuit in which case'the input potential impressed by way of terminals a-b is of constant and fixed frequency and the resonant frequency of the circuit 20-41 is varied such as by varying the inductance 20 or the condenser 2|. The 25 latter may be a condenser microphone or any other mechanically moving element adapted to affect the tuning capacity of the circuit, whereby capacity variations or small mechanical movements may be directly translated into corresponding amplitude variations of electric current supplied from the output terminals c-d in a manner well understood from the above. A simplified arrangement of the latter type for converting variations of inductance or capacity is shown in Figure 5. In the latter, item 35 is a multi-function electron tube comprising a hexode similar to that shown in Figures 1 and'2, while 40 the triode comprises a section of the cathode II, a control grid 31 and a plate 38. The triode section of the tube serves to produce an auxiliary oscillation for which purpose there is provided a tuned circuit comprising an inductance, coil 40 5 on the one hand to the plus terminal of ahigh potential source in series with a voltage drop resistance 44 and on'the other hand to th8 cathode by way of coupling condenser 42. A, grid leak 43 is connected between the grid 31 and the cathode to produce steady biasing po- 65 tential for the grid 31. In this manner, sustained oscillations will be generated in the circuit :Hu' which are impressed upon the input grid" of Y the hexode section such as by directly internally connecting grid 31 of the triode section with the grid I3 of the hexode section. A resonant circuitcomprising an induction coil 48 shunted by a condenser 41 and variable resistance 48 is connected between the grid l2 and ground I! and a neu-.

tralizingcondenser 11 in series with a variable resistance 18 is connected between the input grid 13 and the grid 12 in a manner substantially similar to that described hereinbeior'el 49-50-51 is a filter in the output circuit, 52 a coupling resistsince, 53 a coupling condenser, and 56 and 51 a further voltage drop resistance and shunting condenser, respectively, to supply proper positive potential for the screen grid 14. In operation, the oscillating potential on the grid 13 will cause a "similar potegtial to be developed upon the grid I2 at varying phase depending upon the relative frequency departure 'of the resonant frequency of the tuned circuit 46 -41-48'irom the oscillating frequency determined by the tuning adjustment W; natural frequency of the circuit 46-41-48 as caused by variations of the inductance 46 or the condenser 41 are converted directly into current of varying amplitude obtained from output ter- .minals od.

As will be evident fromthe foregoing the invention has many uses, primarily as a demodulator for frequency modulated radio signals or as a discriminator automatic frequency control arrangements embodied in radio receivers.

Furthermore, the inventive circuit may advantageously be used in numerous electronic control systems involving the problem of converting variations of. an inductance, capacity, or resistanceinto corresponding electric current varia-- tions. In the case of a variable resistance 48 it is seen that a corresponding variation of the output current is obtained due to the change in the conversion constant or steepness of the curves a and b as shown in Figures 3 and 4. In place ofa: parallel tuned resonant circuit connected between the grid I2 and ground, a series tuned circuit or any other resonant network may be employed without departingfrom the spirit of the invention.

Furthermore, it is understood that any suitable type of electron discharge tube may be employed for the purpose of the invention. adapted to produce a space charge coupling and differing in other details from the tubes shown in the drawing for purpose of illustration. Accordingly, it is understood that the invention is not limited to the specific details and arrangementsof parts as well as circuits shown and disclosed herein for illustration, but that the novel principle and underlying thought of the invention are susceptible of numerous variations and modifications coining within the broader scope and spirit of the invention as defined in the ensuing .claims. The specification and drawing are to be regarded in anillustrative rather than in a limiting sense.

I claim: i. 1. In a frequency variation response circuit, an

H electron discharge tube comprising a cathode and an anode, a first control grid near the cathode,

and a second control grid, means'for producing a space charge coupling from said first control grid towards said second control grid,exterior dapaci tative coupling means between said grids to substantially neutralize said space chargecoupling, means for impressing an alternating potential pedance means connected to said first control grid and cathode, the relative frequencyijoi the impressed potential with respect to the resonant frequency of said impedance r neans being varied, an output circuit connected to said anode, and means operatively associated with said output circuitfor developing output energy varying in amplitude proportionately to the frequency departure of said impressedpotential from the resonant frequency of said impedance means.

2. In a frequency variation response circuit,

an electron discharge tube comprising a cathode,

an anode, afirst control near the cathode, and a second control grid, a screen grid located between said first and second control grids, means for maintaining said screen grid at a positive uponsaid second control grid, resonant impotential with respect to said cathode to produce a, space charge coupling from said first control grid towards said second control grid, exterior capacitative coupling means betweensaid grids to substantially neutralize said space ,charge coupling, means for impressing-,analternating potential upon said second control grid, resonant impedance means connected to said first control grid and cathode, the relative frequency of the impressedpotential with respect to the resonant frequency of said impedance meansrbeing'varied, an output circuit connected to said anode, and means operatively associated with said outputcircuit for developing output energy varying in amplitude proportionately to the frequency departure of said impressed potential from the resonant frequency of said impedance means. I

3. In a frequency variation response circuit, anelectron discharge tube comprising a cathode, an anode, a first control grid near the cathode,

and a second control grid, means for producing a space charge coupling from said first control ygrid towards said second control grid, exterior capacitative coupling means between said control grids to substantially neutralize said space charge coupling, means for impressing an alternating potential of varying frequency upon said second control grid and cathode, resonant impe'dance means tuned to a predetermined frequency connected to said first control grid and cathode, an output circuit connected to said anode, and impedance means operatively associated with said output circuit for developing outputenergy varying in amplitude proportionately to the frequency departure of said impressedpotential from the resonant frequency of said impedance means.

4. In a frequency variation response circuit, an

electron discharge tube comprising a cathode, an anode, a first control grid near the cathode and=, a second control grid, means for producing a space charge coupling from said first control grid towards said second control grid, exterior capacitative coupling means between said control grids to substantially neutralize said space charge coupling, means for impressing an alternating potential of constant frequency upon said second control grid and cathode, a tuned circuit connected to said first control grid and cathode,

' control grid towards said second control grid, 9.

series path comprising a'conden'ser and a resistance connected between said control grids to provide an exterior capacitative coupling between said control grids to substantially neutralize said space'charge coupling, means for impressing an alternating potential upon said second grid and cathode, resonant impedance means connected between said first control grid and cathode, the relative frequency of said impressed potential with respect to the resonant frequency of said impedance means being varied, an output circuit connected to said anode, and impedance means operatively associated with said output circuit for developing output energy varying in amplitude proportionately to the frequency departure of said impressed potential from the resonant frequency of said impedance means.

6, In a frequency variation response circuit, an electron discharge tube comprising a cathode, an anode, a, first control grid near the cathode, and a second control grid, means for producing a-space charge coupling from said first control grid towards said second control grid, exterior capacitative coupling means between said control grids to substantially neutralize said space charge coupling, means for impressing an alternating potential upon ,said second grid and cathode, a tuned circuit connected to said first grid and cathode, resistance means for controlling the damping of said tuned circuit, the relative frequency of said impressed potential with respect to the resonant frequency of said tuned circuit being varied, an output circuit connected to said anode, and impedance means operatively associated with said output circuit for developing output energy varying in amplitude proportionately to the frequency departure of said impressed potential from the resonant frequency and cathode, resonant impedance means connected to said first control grid and cathode, the

relative frequency of, said impressed potential" with respect to the resonant frequency of .said

" impedance means being varied, an output circuit connected to said anode, and load impedance means inserted in said output circuit: adapted to develop output energyvarying in amplitude proportionately to the frequency departure of said impressed potential from the resonant frequency of said impedance means.

8. The combination with a source of alternating potential and an electric circuit, of means for exciting said circuit by said alternating potential comprising an electron discharge tube having a cathode followed in the order named by a de- 1 celerating grid, an accelerating grid and a further decelerating electrode, means for maintaining said-accelerating grid at a steady positive potential with respect to said cathode to normally produce-a variable space charge coupling from said first decelerating grid towards said decelerating electrode, means for connecting said source to said decelerating electrode and said cathode, means'for connecting said circuit to said decelerating grid and said cathode, and exterior capacitative coupling means connecting said decelerating grid and said decelerating electrode to substantially neutralize said space charge coupling whereby said circuit is excited by uni-lateral capacitative coupling by said source.

1m ZAKARIAS. 

